1. Field of the Invention
The present invention relates to an acrylic copolymer and, more particularly, to an acrylic copolymer consisting of at least two types of acrylic monomers.
2. Related Background Art
Heretofore, researches have been conducted on acrylic resin which is considered to be excellent organic glass in processability, light transmittance, specific gravity, safety and the like as compared to general glass. In particular, polymethyl methacrylate (PMMA), which is a hard and transparent material with high light transmittance, has been used in applications including materials for automobile components and optical members. However, PMMA is subject to thermal decomposition and offers low heat resistance, incurring limitations in application range.
Meanwhile, a polymer of acrylic ester having acyloxy groups with a small number of carbon atoms at its α position is described in “Conversion-Dependent Molecular Weight of the Polymer in Free Radical Polymerization of Captodative Substituted (Acyloxy)acrylates” (H. Yanaka, et al., Macromolecules, Vol. 30, No. 14, (1997) pp. 4010–4012) (Document 1). However, Document 1 merely reveals relation between the molecular weight of the resulting polymer and the polymerizability of the acrylic ester, relation between the molecular weight of the resulting polymer and polymerization conditions, and the like. Accordingly, an elastic modulus, heat resistance and mechanical properties of the resulting polymer have not been revealed. In this connection, the present inventors prepared and evaluated the polymer described in Document 1. In this study, they confirmed that the resulting polymer has an elastic modulus (an elastic modulus in typical working temperature range, hereinafter referred to as “a normal temperature elastic modulus”) equal to that of PMMA and offers higher heat resistance (an elastic modulus and resistance to thermal decomposition at higher temperatures, hereinafter collectively referred to as “heat resistance”) than PMMA. However, the polymer described in Document 1 hardly softens below the thermal decomposition temperature, and therefore still remains unsatisfactory in that general molding methods cannot be used for molding.
Moreover, in U.S. Pat. No. 2,559,635 (Document 2) and “Radical copolymerization of methyl α-acyloxyacrylates with some vinyl monomers” (T. Hongo, et al., Polymer International, Vol. 48 (1999) pp. 505–508) (Document 3), there is described a copolymer of acrylic ester having acyloxy groups with a small number of carbon atoms at its α position and vinyl monomer such as styrene, methyl methacrylate and acrylonitrile. However, since Documents 2 and 3 have also not revealed the elastic modulus, heat resistance and mechanical properties of the resulting copolymer, the present inventors prepared and evaluated the copolymer described in these documents. In this study, they found out that the resulting copolymer offered reduced normal temperature elastic modulus and heat resistance compared to the polymer described in Document 1, and therefore concluded that this copolymer still remains unsatisfactory.